1. Field of the Invention
The present invention relates to an optical disk unit, including a CD-ROM, CD-R or CD-RW drive, a DVD drive, or a Blu-Ray Disc (trademark) drive, a control method for such a disk unit, and a computer readable storage medium.
2. Description of the Related Art
Various types of optical disk medium have been developed in recent years, and they are appropriately used depending on the purpose. Generally, optical disk media have a structure with a plurality of layers arranged on top of one another. That is, regardless of the type of optical disc medium, protective layers are provided on both surfaces of the medium, and data storage layers for storing signals are formed enclosed by the protective layers. However, many parameters are different, such as thickness of the optical disk medium itself, distance from a surface of the protective layer to a surface of a data storage layer (signal surface), number of signal surfaces (for example, with a DVD, there is a maximum of two signal surfaces), and wavelength of a laser to be used in reading out information from the signal surface.
For this reason, it is normal to use a dedicated drive for each type of optical disk medium. However, with the need to purchase and install a dedicated drive for each type of optical disk, the user must master the operations of each drive, and the financial burden is also significant. There has therefore been a demand for a drive (optical click drive) that can handle many types of optical disk media.
For a drive to handle these many types of optical disk media, technology is being developed to use a different wavelength of light source (laser) used in reading out information for each type of optical disk medium, using an optical element having wavelength selection characteristics in a single objective lens, and changing a numerical aperture of the lens.
An optical pickup 1 for such a drive, as exemplarily shown in FIG. 8, is constructed including a light-emitting element 11 for outputting laser light of a plurality of wavelengths, a beam splitter 12, a photodetector 13 and an objective lens body 14. Also, the objective lens body 14 is comprised of an objective lens 14L and a hologram element 14H including a diffraction grating.
The light-emitting element 11 is a semiconductor laser element for outputting laser light of, for example, three mutually different wavelengths (a so-called three-wavelength laser). The three wavelengths here are controlled so that in the case of handling, for example, a Blu-ray disk, a DVD (Digital Versatile Disk) and a CD (Compact Disk), laser light is output having a wavelength of 405 nanometers for the Blu-ray disk, 650 nanometers for the DVD, and 780 nanometers for the CD.
The beam splitter 12 guides light output by the light emitting element 11 to the objective lens body 14. This beam splitter 12 also guides light that is input by being reflected by the optical disk body and passing through the objective lens body 14 to the photodetector 13. The photodetector 13 is provided with a plurality of light detection elements arranged in a matrix of N×N, for example. This photodetector 13 is also provided with a cylindrical lens, for example for measuring beam diameter. Light that has been guided by the beam splitter 12 reaches the respective plurality of light receiving elements by way of this cylindrical lens. The photodetector 13 then respectively outputs signals for strength of light that has been respectively detected by the plurality of light receiving elements.
The hologram element 14H of the objective lens body 14 diffracts laser light that has been guided by means of the objective lens 14L and reflected by the medium so as to become a predetermined numerical aperture (NA) for each wavelength of laser light, and guides the diffracted laser light to the beam splitter 12. Also, the objective lens 14L is an aspherical lens, and refracts and outputs light that has been guided from the light emitting elements through the beam splitter 12 and the hologram element 14H so that focal points are focused at positions specified by focal distance F that is different for each wavelength. This objective lens 14L also condenses laser light that has been reflected by the medium and guides it to the hologram element 14H laser.
A signal representing focus error of laser light on the storage surface of the optical disk medium (focus error signal; FE signal) and a signal equivalent to a sum of strengths of light that has reached the light receiving elements (pull-in signal; PI signal) are generated from a signal (RF signal) output by the photodetector 13. It is also standard practice to generate a signal representing tracking error (TE signal) etc. from the signal output by the photodetector 13, but detailed description thereof has been omitted here.
Here, the PI (pull-in) signal is a signal shown as (a) in FIG. 9. Specifically, this PI signal has a peak at a positioned where focus is optimum. Also, the FE signal is shown as (b) in FIG. 9. Specifically, the FE (focus error signal) becomes substantially “0” when focus is achieved. Also, when the distance between the optical disk medium changed around a position where focus is achieved, the FE signal has positive and negative peaks respectively at points of certain distance from a position where focus is achieved, and the FE signal crosses zero (crosses a reference position) at a position where focus is achieved.
In FIG. 9, an example is shown of each signal in the case where the objective lens 14L of the optical pickup 1 is moved in a direction approaching the optical disk medium surface, starting at a position separated from the optical disk medium. When light that has been reflected by the optical disk medium surface arrives at the focal point in the optical pickup 1, a peak occurs in the PI signal (S) due to the surface reflection, as shown in (a) of FIG. 9. If the objective lens 14L of the optical pickup 1 is brought closer to the surface of the optical disk medium, the surface reflection light becomes stray light inside the optical pickup 1, and this stray light is detected as a fake signal (Fake). This fake signal is not limited to one signal, and can also be a plurality of signals. If the optical pickup 1 is brought closer to the disk surface, reflected light (T) at the signal surface is detected.
Similarly, for the FE signal which is shown as (b) in FIG. 9, at positions where the surface reflection (S), fake signal (Fake) and reflected light (T) at the signal surface are respectively obtained, a signal representing that an image has been formed is detected.
With the optical disk unit, it is possible to readout signals corresponding to the plurality of optical disk media by controlling the distance between the objective lens body 14L and the medium surface so that a distance from a planar section P of the objective lens 14 to the signal surface inside the medium becomes the focal distance F, that is, so that it is possible to achieve focus on the signal surface. Here, whether or not focus is achieved is determined using the FE signal and/or PI signal, and in focus is determined, for example, when an absolute value for the FE signal exceeds a first threshold level (FZC1), but is less than a second threshold value (FZC2) (approaches “0”). In focus is also determined when the PI signal exceeds a specified threshold value.
An example of an optical disk unit that uses such an optical pickup is disclosed in Japanese patent application 2986587.
Incidentally, depending on the optical disk medium, there are cases where the signal surface of the disk is inclined along a radial axis direction. In such a case, a distance from the objective lens to the signal surface is changed at the rotation cycle accompanying rotation of the optical disk medium. This phenomenon is called axial runout.
If axial runout occurs, there are cases where a fake signal (Fake) appears repeatedly a plurality of times in the PI signals and FE signals, and a result (F2, F3) of the fake signal (FAKE) constituting substantially the same peak level repeatedly appearing is that there are cases where discrimination between a reflected light (T) at the signal surface where focus should actually be achieved and a fake signal appearing due to axial runout is difficult (FIG. 9).
Therefore, attention has focused on the fact that the level of reflected light (T) at the signal surface is generally higher than the fake signal (Fake), and a method has been considered where a threshold value that is a higher level than the level of a fake signal (Fake) that would be expected due to the occurrence of axial runout is set experimentally. For example, the threshold value is set by writing to a read only memory (ROM) at the time of leaving the factory etc., and detection of a reflection at the signal surface (FOK, FZC1 shown by the dotted line in FIG. 9) when a signal exceeds this threshold value.
However, as shown in FIG. 10, the level of a signal detected by the optical pickup becomes low overall due to variations in reflectance of the optical disk medium inserted, dust attached to the optical pickup, or environmental variations, such as temperature. When the level of a signal detected by the optical pickup becomes low, in the case of where the predetermined threshold values FOK, FZC1 are set as described above and fixed values are always used, there may be cases where the level of reflected light at the signal surface does not reach this threshold value and it will be difficult to achieve focus at the signal surface.